Update TransformHelperGen hierarchy and particularly TransformHelper<FPP,GPU2> (new methods).

Transform<FPP,GPU2> new methods: iterator::operator<(iterator), operator+, operator-, multiply(const FPP&).
TransformHelper<FPP,GPU2> new functions: ctor TransformHelper(...), pop_front, pop_back, push_first, get_product(MatDense<FPP,GPU2>&), multiply(const FPP&), (empty, todo) pack_factors, begin, end.
Move up PackDir in TransformHelperGen.
Minor update in test_transform_helper_gpu_mod.cpp.in (renaming to display).